How does the “#map(&proc)” idiom work when introspecting module classes?

Question

Presenting the Idiom

I found an interesting but unexplained alternative to an accepted answer. The code clearly works in the REPL. For example:

module          


        

Answer 1

&Foo.method(:const_get) is the method const_get of the Foo object. Here's another example:

m = 1.method(:+)
#=> #
m.call(1)
#=> 2
(1..3).map(&m)
#=> [2, 3, 4]

So in the end this is just a pointfree way of saying Foo.constants.map { |c| Foo.const_get(c) }. grep uses === to select elements, so it would only get constants that refer to classes, not other values. This can be verified by adding another constant to Foo, e.g. Baz = 1, which will not get grepped.

If you have further questions please add them as comments and I'll try to clarify them.